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  Java Practices -> Compile regular expressions once
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<div class='page-title'>Compile regular expressions once</div>

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<br><tt><a href="http://java.sun.com/javase/6/docs/api/java/util/regex/Pattern.html">Pattern</a></tt>
objects compile the regular expressions which are passed to them. If a
regular expression is used many times, then this compilation should be
performed only once.
<p>Be wary of the these convenience methods, which compile a regular expression
each time they are called:
<ul>
<li>
<tt>Pattern.matches</tt></li>

<li>
<tt><a href="http://java.sun.com/javase/6/docs/api/java/lang/String.html">String</a>.matches</tt></li>

<li>
<tt>String.replaceAll</tt></li>

<li>
<tt>String.replaceFirst</tt></li>

<li>
<tt>String.split</tt></li>
</ul>
<b>Example</b>
<p>Here is a comparison of the performance of <tt>String.matches</tt> versus
the use of <tt>Pattern</tt> and <tt><a href="http://java.sun.com/javase/6/docs/api/java/util/regex/Matcher.html">Matcher</a></tt>.
<p>An example run gives :
<p><tt>Time for String: 70 ms</tt>
<br><tt>Time for Pattern: 10 ms</tt>
<br>
<PRE>

<span class='keyword'>import</span> java.util.*;
<span class='keyword'>import</span> java.io.*;
<span class='keyword'>import</span> java.util.regex.*;

<span class='keyword'>public</span> <span class='keyword'>final</span> <span class='keyword'>class</span> FindMatch {

  <span class='comment'>/**
  * Count the number of times the word "assert" occurs in a file.
  *
  * Compare the execution times of two techniques.
  */</span>
  <span class='keyword'>public</span> <span class='keyword'>static</span> <span class='keyword'>void</span> main (String... aArguments){
    FindMatch findMatch = <span class='keyword'>new</span> FindMatch(<span class='literal'>"assert"</span>);
    File file = <span class='keyword'>new</span> File(<span class='literal'>"C:&#092;&#092;Temp&#092;&#092;InheritanceInterpreter.java"</span>);

    Stopwatch stopwatch = <span class='keyword'>new</span> Stopwatch();
    stopwatch.start();
    findMatch.countMatchesUsingString(file);
    stopwatch.stop();
    log(<span class='literal'>"Time for String: "</span> + stopwatch);

    stopwatch.start();
    findMatch.countMatchesUsingPattern(file);
    stopwatch.stop();
    log(<span class='literal'>"Time for Pattern: "</span> + stopwatch);
  }

  <span class='keyword'>public</span> FindMatch(String aTarget){
    fTargetWord = aTarget;
    fPattern = Pattern.compile(fTargetWord);
  }

  <span class='keyword'>public</span> <span class='keyword'>int</span> countMatchesUsingString(File aFile){
    <span class='keyword'>int</span> result = <span class='literal'>0</span>;
    <span class='keyword'>try</span> {
      Scanner scanner = <span class='keyword'>new</span> Scanner(aFile);
      <span class='keyword'>while</span> ( scanner.hasNextLine() ){
        String line = scanner.nextLine();
        <span class='keyword'>if</span>( line.matches(fTargetWord) ){
          ++ result;
        }
      }
    }
    <span class='keyword'>catch</span> (FileNotFoundException ex) {
      log(ex);
    }
    <span class='keyword'>return</span> result;
  }

  <span class='keyword'>public</span> <span class='keyword'>int</span> countMatchesUsingPattern(File aFile) {
    <span class='keyword'>int</span> result = <span class='literal'>0</span>;
    Matcher matcher = fPattern.matcher(<span class='literal'>""</span>);
    <span class='keyword'>try</span> {
      Scanner scanner = <span class='keyword'>new</span> Scanner(aFile);
      <span class='keyword'>while</span> ( scanner.hasNextLine() ){
        String line = scanner.nextLine();
        matcher.reset( line ); <span class='comment'>//reset the input each time
</span>        <span class='keyword'>if</span> ( ! matcher.find() ) {
          ++result;
        }
      }
    }
    <span class='keyword'>catch</span> (FileNotFoundException ex) {
      log(ex);
    }
    <span class='keyword'>return</span> result;
  }

  <span class='comment'>// PRIVATE //
</span>  <span class='keyword'>private</span> <span class='keyword'>final</span> String fTargetWord;
  <span class='keyword'>private</span> <span class='keyword'>final</span> Pattern fPattern;

  <span class='keyword'>private</span> <span class='keyword'>static</span> <span class='keyword'>void</span> log(Object aObject){
    System.out.println(aObject);
  }
} 
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